3*1 



GLASS HAVUFACrUBK. 



GLASS MANUFACTURE. 



home-make. Tbo oarliart manufacture of flint-gla** in England waa 

 begun in 1357 ; and the program made in perfecting it waa no alow, 

 that it waa not until near toe cloae of the 17th century that this 

 country waa independent of foreigner* for the supply of the oomiuou 

 article of driuking glimni. In 1078 aome plate-glass waa made at 

 Lambeth, in work* supported by the Duke of Buckingham, but hi. li 

 were aoon abandoned. It waa exactly one century later that the first 

 establishment of magnitude for the production of plate-glas* waa 

 formed in this country, under the title of " The Qovernor and Com- 

 pany of British Cast Plate-glass Manufacturers." The members of this 

 company subscribed an ample capital, and works upon a large scale 

 wen erected at Ravenhcnd, near Prescot in Lancashire, which hare 

 been in constant and successful operation from that time to the 

 present day. 



There are several distinct kinds of glass, which differ from each 

 other in regard to some of the ingredient* of which they are made, 

 and in the proceeses of manufacture. The names crotnt, Jlint, cut, win- 

 dow, Aett, broad, tprtad, blown, coil, plate, Ac., glass, are not very well 

 chosen ; but whatever be the names, the principal ingredients employed 

 are silex or flint, and an alkali. The differences in the various kinds 

 result from the description of alkali employed, and from the addition 

 of certain accessary materials, usually metallic oxides. The form in 

 which silex is now generally used in this country for glass-making is 

 that of sea-sand, and care is required to select those kinds which are 

 free from foreign matters and impurities. The port of Lynn in Nor- 

 folk, and Alum Bay in the Isle of Wight, have long furnished the 

 greater part of the silex used in our glass-houses. Flint-glass derives 

 its name from the practice in former times of using flints calcined and 

 ground iu the manner now employed for making porcelain ; but this 

 has long been discontinued. Of late there has been some apprehension 

 of a scarcity of sand suitable to the manufacture ; and a good idea- may 

 be formed as to the importance attached to the purity of this chief 

 ingredient from the fact that sand has been imported for the purpose 

 from New South Wales. The alkali employed for making fine flint- 

 glass is pearl-ash, purified by solution and subsidence, in which process 

 impurities to the extent of one-third of the weight are removed. 

 Coarser alkaline substances are used for making inferior kinds of glass : 

 the impurities even assist towards fusing the silex ; but such alkalies 

 all contain iron in some degree, and it is to the presence of this metal 

 that the green colour of common glass is owing. Barilli and kelp 

 were formerly the chief sources of the soda employed ; but in recent 

 years the carbonate and sulphate of this alkali have been obtained 

 from common salt ; either one alone, or the two combined, being 

 chosen for different kinds of glass. 



We shall now treat succinctly of the processes by which the prin- 

 cipal kinds of glass are produced. 



I- lint lilatt. This, known in other countries under the name of 

 crystal, is the most generally useful, the most brilliant, and the heaviest 

 description of glass. This last quality it owes to the large quantity of 

 oxide of lead which it contains, and which is used sometimes in the 

 form of minium, but more frequently in that of litharge. The metallic 

 oxide acts as a flux, and promotes the fusion of the other materials 

 at a comparatively low temperature. The greater density which it 

 imparts give* to the glass a greater power of refracting the rays of 

 light ; and it is this quality which renders flint-glass of so much im- 

 portance for optical purposes. Nitre in a small proportion is used for 

 the destruction of any carbonaceous matter in the other ingredients ; 

 the oxygen which it gives out in the furnace further serves to maintain 

 at their highest degree of oxygenation the metallic oxides that are 

 present Black oxide of manganese in minute proportion is also used 

 to remove any foul colour that might otherwise remain through the 

 impurity of the alkali used; its cleansing property occasioned this 

 oxide to be known formerly under the name of glass-soap. Any undue 

 proportion of manganese would impart a purple hue to the mass ; and 

 if any considerable quantity be used that colour will be deepened 

 almost to black. When through inadvertence the glass has been made 

 purple, the colour will be almost instantly discharged by thrusting a 

 piece of wood into the melted maas. The cause of these changes is as 

 follows : The purple colour given by oxide of ninnginimfi arises from 

 iU being in a high state of oxygenation ; the wood when thrust into 

 the heated mast becomes speedily carbonised, and the carbon, com- 

 bining with the superfluous oxygen, is driven off in the form of carbonic 

 acid gas ; if by the addition of nitre the quantity of oxygen is again 

 increased, it will combine with the manganese, and restore the purple 

 colour. It will be seen from these circumstances how much skill and 

 experience are necessary for the due mixture of ingredients so as to 

 produce glass of the best quality. The manufacturers of flint-glass 

 are generally unwilling to disclose the precise proportion* in which 

 they employ the requisite ingredients ; and it i probable that each 

 ha* a favourite recipe of hi* own. One kind consists of 120 parts fine 

 clean white sand, 4u well-purified pearl ash, 85 litharge or minium, 13 

 nitre, and a small (undefined) quantity of the black oxide of manga- 

 neee. The French chemists recommend a much larger proportion of 

 oxide of lead, but this is found to make the glass inconveniently soft. 

 Where leu nM^'k oxide is used, more nitre is required as a flux, and 

 rite vena. 



The ingredient*, in whatever proportion* selected, must all be inti- 

 mately mixed before they are put into the crucibles or pot*, which 



are previously placed in the furnace. These pot* are very carefully 

 made of Stourbridge clay ; they are about a yard in height, with only 

 one arch-abaped opening at the aide, and each will contain 16 cwt. of 

 glaas. A* the bulk decreases by fusion, fresh portions of the ingre- 



Mclting-poU fur Flint-glut. 



dients are added until the pots are full of melted glass. A very strong 

 and long-continued heat is necessary, not only for the perfect : 

 and amalgamation of the materials, but also for the discharge of the 

 impurities which they contain. The chief of these, known under tli- 

 name of sandivir, or glass-gall, consists of salts existing in the alkali 

 which have but small affinity for silex, and from their specific levity 

 rise in the form of a white porous scum to the top of the crucible, 

 whence it must be removed before it is volatilised by the excessive 

 heat of the furnace. This glass-gall is used as a powerful flux by 

 refiners of metals. When the whole of the impurities have been 

 thus thrown off by the action of heat and are removed, and the 

 glass, or metal, as it is called, appears colourless and translucent, 

 the vitrification is known to be complete. The temperature of the 

 furnace is then lowered by preventing the access of air until the glass 

 loses a part of its fluidity, and. assumes that pasty character which 

 is the most convenient for the workmen ; it being sufficiently consistent 

 to be tenacious, but soft enough to yield to the slightest pressure with- 

 out cracking or losing its tenuity. The material is usually brought to 

 a perfect state of vitrification in about forty-eight hours from the first 

 application of heat. 



There is perhaps no process of manufacture which more excites the 

 surprise and admiration of a stranger than that of fashioning flint-glass 

 into all the various objects of convenience and ornament for whifh ii 

 is employed. To see a substance, proverbially brittle. Mown with the 

 human breath, pulled, twisted, cut, and then joined again with tin 

 greatest facility, never fails to strike with astonishment those who are 

 unaccustomed to the sight. The tools with which all these operations 

 are performed are of the most iuartiKri.il description, and do not appear 

 to have received any improvement from the earliest records of the 

 manufacture. Most articles in flint-glass, such as drinking glasses, 

 cruets, decanters, phials, Ac., ore made by the combined processes of 

 blowing, manipulation, and casting, in varying degrees. A square 

 perfumery bottle may be selected a* an example of one kind. The 

 workman takes a long iron tube, and collects a little of the melted glaas 

 on one end of it, by thrusting the tube through the open door of the 

 furnace into the mouth of the melting-pot ; this pasty glass is rolled 

 into a cylindrical form on an iron plate, and is pinched at one place 

 to form a neck. The glass is then thrust into a brass or iron mould, 

 and the workman blows through the other end of the tube; the 

 mould gives an external shape to the glass, while the current of 

 breath hollows the interior. All this is done in less than half a minute. 

 The mould, which consists of two halves hinged together, is opened ; 



Mouldt for calling Flint-gUu bottle*. 



the glass is taken out ; it is detached from the tube by a touch with 

 a piece of cold iron ; it is taken up on the end of a heated rod by 

 another workman ; and by mean* of a few simple tools the neck 

 and mouth of the bottle are finished. A claret-jug may be taken as 

 an example of another large class of flint-glass manufactures. The 

 workman collects a considerable quantity of melted glaas on the end 

 of hi* iron tube, which is railed to elongate it, rolls it into a cylin- 



